Researchers use a wide range of methods to monitor the Baltic Sea, with each of these having its own strengths and limitations. For instance satellites can only observe the sea's surface. Meanwhile, monitoring stations provide information from just a single point, while research vessels are expensive, and their measurements are conducted infrequently. Underwater gliders on the other hand cannot operate at depths of over approximately 30 meters.

As a result, a significant observational gap has emerged in coastal waters and straits. Over the past year, scientists testing a French-made autonomous robot known as YUCO-PHYSICO have been aiming to plug this gap. According to TalTech senior researcher Taavi Liblik, the team carried out 11 successful test missions in three locations: Matsalu Bay on the west coast, the Suur Strait (Suur väin) between Saaremaa/Muhu and the mainland, and out on the open Baltic Sea.

"The device has excellent maneuverability. In Matsalu Bay, we even programmed it to conduct measurements between fishing nets," Liblik noted.

The robot is a fairly autonomous assistant, he added. "We program the route in advance, go to the starting point, and put it in the water. Then we give it a start command from the control panel, and it starts its mission. Last year we were following close behind to maintain radio contact and to check all was going smoothly. In future, however, the likely scenario is that you would put it in the water in the morning and retrieve it that evening," he added.

The device weighs only 10 kilograms and is just over a meter long; it can measure water temperature, salinity, oxygen content, turbidity, and chlorophyll, an indicator of plankton abundance.

There may be scope for surveying for cyanobacteria – commonly known as blue-green algae, which often blights some of Estonia's beaches in summer.

"At the moment, we do not have a phycocyanin sensor for detecting cyanobacteria, but in principle one could be attached. It could also be sent into an oil slick and allowed to drift with it. That would provide information about where the oil slick is moving. In the future, it will probably be able to handle highly flexible tasks," Liblik explained.

The test missions revealed several interesting findings, Liblik went on. For example, scientists were able for the first time to observe in detail how water from the open Baltic Sea moved through the Väinameri archipelago sea, between Estonia's largest islands, and penetrated the Gulf of Riga to a set depth. Meanwhile off Osmussaar Island in the Gulf of Finland to the north, the team also discovered an unusual upwelling event.

"Upwelling is what makes our coastal waters cold in summer, when water rises to the surface from an intermediate layer about 40 meters deep," Liblik explained.

This particular finding was interesting to researchers because of its unusual makeup. "There was some completely cold water along the shore, followed by a somewhat warmer zone, while then, a few kilometers farther away, another similar cold-water zone. It turned out that the seabed topography there was responsible, and the device mapped the structure, providing an entirely new and detailed picture," he added.

The video below shows the AUV being launched.

Having demonstrated its capabilities in the Baltic Sea, the robot is already being used in a new project. This time, scientists are studying the marine environment of the Irbe Strait, where the Gulf of Riga opens out into the wider Baltic.

"The Irbe Strait is a critically important area for the Gulf of Riga. Previous measurements from deeper layers show that oxygen deficiency there may last only a week in some years, while in others it can persist for several months. Now we want to use the robot to see how water masses move through the strait."

In Liblik's view, devices such as the one being tested will become an indispensable part of marine observation going forward in a wide range of scenarios.

"There are many options. Since it is a small device designed for one-day missions, that makes it an excellent tool for responding to events occurring at sea. It is also suited to monitoring development projects, for example if offshore wind farms are eventually built here," he explained.

Liblik added that the device is so quiet that it can also be used in nature reserves, where scientists may prefer not to travel even by motorboat.

In addition to the researchers from TalTech, the ongoing project also involves scientists from Bulgaria and Turkey, who are carrying out similar measurements in the Black Sea; Japanese researchers, too, have been using the robot to monitor conditions in the Sea of Japan.

Read more about the test missions in the scientific journal Frontiers in Marine Science.

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